One essential task in the reproduction of color images is retouching the image after it has been scanned to a digital storage medium by means of a scanning device such as the Scitex Smart 342L Scanner, manufactured by Scitex Corporation of Herzlia, Israel. Retouching is a process for digitally correcting defects in the image caused by scratches in the original or dust, hairs and other particles that have adhered to the original or to the scanning device prior to scanning. The most popular method of retouching is by interactive manual means with the aid of a computer processor, a monitor display, an electronic pointing and clocking device such as a mouse connected on a bus to the computer processor and a suitable software program. In this method, the image is displayed on the monitor and the operator uses the pointing and clicking device to select non-defective pixels from an area adjacent to the defective pixels, said area being of similar color and similarly textured to the defective area in the uncorrupted original. The non-defective pixels are copied over the defective pixels by the software program. The corrected image is then stored in place of the original on the digital storage medium. The disadvantages of this method are the length of time required to perform it and the fact that it often requires a skilled operator. All areas of the image have to be closely inspected in the search for defects and the selection of non-defective pixels to be copied over defective pixels must be skillfully made in order to maintain a natural appearance after correction of the defect.
Another method of retouching is by use of a digital filter such as the "dust and Scratches" filter to be found in the software program "Photoshop 3.0", marketed by Adobe, Inc, of Mountain View, Calif., USA. This program enables the operator to select an area to be corrected and to automatically retouch objects in the selected area which are narrower than an operator selected radius and of greater contrast from their surrounding than an operator selected threshold. Objects satisfying the thresholds are given new color values taken from the surrounding area. One disadvantage of this method is that a connected object may be partially retouched at its narrowest point and left defective at its widest point. Another disadvantage is that objects slightly larger than the selected radius may be partially retouched around their circumference and thereby made thinner. Yet another disadvantage of this method is the difficulty of guarding actual scene objects satisfying the thresholds against undesired retouching. The operator has to guess in advance all objects that may be unwillingly retouched in order to deselect the area including them or else he has to undo the already performed retouch corrections.
U.S. Pat. No. 4,484,230 discloses an image reproduction method and apparatus in which areas in the image are marked either by fluorescent marking on the original or by digital means on a representation of the image on a monitor, and the defects contained in the marked areas are detected and corrected during final image reproduction. Detected defects cannot be examined and authorized by the operator because detection and correction of defects are effected simultaneously during the scanning process. The operator must successfully indicate all defect areas in advance by inspection of the image and cannot correct errors without rescanning the original. Defects are treated on a pixel by pixel basis and at no stage are they linked into connected segments for treatment as distinct objects.
In most cases, no set of parameters perfectly separates defects from similar, real image elements. This results both in false correction of image elements and in missed correction of defects.